Connect a voltmeter between the positive and negative poles of the battery.
Start the engine. When starting the engine, the battery voltage may drop to V volts (at ambient temperature +20°С).
Increase the engine speed to 3000 rpm. In this case, the voltage should be from 13.5 to 14.5 volts. This is proof that the generator and relay-regulator are working properly. To ensure recharging during operation, the generator voltage (onboard voltage) must be higher than the battery voltage.
Check the stability of the voltage regulation. To do this, turn on the main beam and repeat the measurement at an engine speed of 3000 rpm. The measured voltage value should not exceed the previously obtained value by more than 0.4 volts.
If the measured voltage values are outside the prescribed values, the generator must be checked by a specialist workshop.